I just bought a used TREK 830 mountain bike with a shimano 21 speed. It rides alright, but the only conundrum is somtimes when shifting (scull shifter) it takes some gear grinding to shift into gear. Also when I stand up to pedal hard the bike goes out of gear and into another gear. How do I fix this?
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take it to a shop have them chek the chain, cassette & chainrings for wear. also have the derailleurs adjusted as needed. to adjust the front derailleur turn the adjuster barrel on the shifter where the cable housing enters. for the rear der adjuster is on the back of the der where the cable housing enters.
you need to adjust the derailleurs. this can get complicated so i recommend you take it in for a tune up. if you want to try it yourself first (all you can do is make it worse and then take it in for a tune up) you can try adjusting it. Place it in the highest gear, the smallest in the back and turn the barrel adjuster. shift to the next gear and back down. keep adjusting until it shifts right between the bottom and next gear. if you go too far in one direction, you won’t be able to shift to the bottom gear. if you go too far in the other direction, the bike won’t shift on the first click of the shifter.
It is a simple shifter cable adjustment. There is a barrel adjuster where the cable enters the rear derailleur. It will have small nubs on it and it screws in and out.
If it is shifting late going into a lower gear turn it out 1/2 turn. If the bike shifts into a higher gear when you pedal hard turn it out 1/2 turn.
If the shift is late going into a higher gear or it shifts to a lower gear under hard pedaling turn it in 1/2 turn.
When it is close to shifting correctly 1/4 turn will fine tune it.
Cleaning and lubing your shift cables will also help.
Do not adjust any of the screws on the derailleur.
http://bicycletutor.com/adjust-rear-derailleur/
A excellent chain and cable lube is a must.
http://bicycletutor.com/lube-brake-shift-cables/